Delignification and bleaching of cellulose pulp with oxygen gas in an allsaline medium in the presence of a protector

ABSTRACT

A method of delignifying and bleaching a chemical or semichemical cellulose pulp with oxygen gas in an alkaline medium, preferably sodium hydroxide, and in the presence of a protector, the method being characterized in that the protector is selected from the group consisting of silica; an alkali metal silicate, such as sodium silicate; an alkaline earth metal silicate, such as magnesium silicate; an alkaline earth metal phosphate, such as magnesium phosphate; an alkaline earth metal oxide, such as magnesium oxide; an alkaline earth metal peroxide; an alkaline earth metal hydroxide or a material forming an alkaline earth metal hydroxide, such as magnesium hydroxide or a material forming magnesium hydroxide in situ; and a soluble salt of magnesium other than magnesium carbonate.

United States Patent Smith et al.

[15] 3,657,065 [451 Apr. 18, 1972 Inventors:

Assignees:

Filed:

Appl. No.:

Leon Smith, Petersfield Springs; David W. Calvert, Westdene, Benoni, both of Republic of South Africa; Andre Robert, .Meylatl; n r all 992921291299 9 France South African Pulp and Paper Industries, Limited; LAir Liquide, Societe Anonyme pour LEtude et LExploitation des Procedes Georges Claude June 11, 1969 Foreign Application Priority Data June 13, 1968 Republic ofSouth Africa ..68/3771 US Cl ..l62/65, 162/70, 162/78,

162/80, 162/90 Int. Cl ..D21c 9/10 Field of Search 162/65, 70, 78, 80, 90

[56] References Cited UNITED STATES PATENTS 1,887,863 11/1932 Stevenson ..162/90 X 2,169,473 8/1939 Olsen ....162/90 X 3,382,149 5/1968 Hoh ....l62/65 X 3,384,533 5/1968 Andre et al. ..162/65 Primary Examiner-S. Leon Bashore Assistant Examiner-Arthur L. Corbin Att0rneyWaters, Roditi, Schwartz & Nissen ABSTRACT A method of delignifying and bleaching a chemical or semichemical cellulose pulp with oxygen gas in an alkaline medium, preferably sodium hydroxide, and in the presence of a protector, the method being characterized in that the protector is selected from the group consisting of silica; an alkali metal silicate, such as sodium silicate; an alkaline earth metal silicate, such as magnesium silicate; an alkaline earth metal phosphate, such as magnesium phosphate; an alkaline earth metal oxide, such as magnesium oxide; an alkaline earth metal peroxide; an alkaline earth metal hydroxide or a material forming an alkaline earth metal hydroxide, such as magnesium hydroxide or a material forming magnesium hydroxide in situ; and a soluble salt of magnesium other than magnesium carbonate.

10 Claims, No Drawings DELIGNIFICATION AND BLEACI-IING OF CELLULOSE PULP WITH OXYGEN GAS IN AN ALLSALINE MEDIUM IN THE PRESENCE OF A PROTECTOR This invention relates to the delignification and bleaching of chemical and semi-chemical cellulose pulp with oxygen gas in alkaline medium.

For the purposes of this specification, the term oxygen gas includes any other gas containing free oxygen, such as air.

It is known to delignify and bleach cellulose pulp with oxygen gas in an alkaline medium and excellent delignifying and bleaching effects are obtained if this treatment is carried out at elevated temperatures near 100 C and under elevated pressures of the order of 70 145 p.s.i.g. However, this conventional treatment suffers from the disadvantage that at such elevated temperatures and pressures, the physical and mechanical strength properties of the pulp are affected adversely.

In our French Patent Specification No. 1,387.853, we disclosed that by treating a chemical or semi-chemical pulp with oxygen in an alkaline medium in the presence of certain catalysts or protectors, the physical and mechanical strength properties of the pulp can be preserved, and in some cases even improved, while giving a very complete delignification and a significant increase in brightness. More particularly, we disclose a method for the delignification and bleaching of chemical and semi-chemical cellulose pulp with oxygen in an alkaline medium characterized in that the treatment of the pulp is effected in the presence of a protector used at a concentration from 0.5 to about 3 percent by weight on the pulp, selected from the group consisting of barium carbonate, calcium carbonate, magnesium carbonate, zinc carbonate, alkali metal borates and titanium dioxide.

It has now been found that the catalyst or protector may be selected from a new group. Many of these new protectors have the great advantage of more efiectively preserving the physical strength properties of cellulose pulps, and even of improving them in some cases, than certain of the known protectors, while giving a significant increase in brightness.

According to the present invention, the protector may be selected from the group consisting of silica; and alkaline metal silicate, such as sodium silicate; an alkaline earth metal silicate, such as magnesium silicate; an alkaline earth metal phosphate, such as magnesium phosphate; an alkaline earth metal oxide, such as magnesium oxide; an alkaline earth metal peroxide, such as magnesium peroxide; an alkaline earth metal hydroxide or a material forming an alkaline earth metal hydroxide, such as magnesium hydroxide or a material forming magnesium hydroxide in situ; and a soluble salt of magnesium, such as magnesium acetate or magnesium chloride.

Sodium silicate, magnesium oxide, magnesium hydroxide and magnesium silicate are particularly advantageous.

The new protectors give the best results when used at a concentration from 0.5 to about 3 percent preferably 1 percent, by weight of the pulp on a dry basis.

Any suitable alkaline medium may be used, preferably sodium hydroxide. The sodium hydroxide content may vary from 2 to 12 percent by weight on a dry pulp basis depending on the degree of delignification and bleaching that is required.

Generally speaking, the quantity of sodium hydroxide required may be increased for untreated pulps with a high lignin content.

The oxygen gas treatment is best carried out in any suitable pressure apparatus which will ensure intimate contact or adequate exposure of the pulp mass with or to oxygen in the gas phase.

During oxygen gas treatment, the pulp consistency may be in the range from 4 to 30 percent by weight, preferably 7 to 25 percent percent.

In a preferred embodiment of the invention, an untreated alkaline pulp is treated with sodium hydroxide in the presence of the protector with oxygen gas in a pressure vessel at an elevated operating temperature, preferably between 90 and C, and at an elevated partial pressure at the maximum operating temperature, preferably in the range 70 to p.s.i.g. The pressure need only be limited by the characteristics of the apparatus used. The pulp may be subjected to the oxygen treatment at the maximum operating temperature for a period of time ranging from 10 to 120 minutes, depending on the results required or the nature of the pulp being treated.

After the oxygen treatment, the pulp may be washed in any suitable manner. The choice of the washing operation is largely governed by whether recovery of chemicals in the effluent or subsequent bleaching is contemplated. It is advantageous for the washing operation to be done carefully and for the effluent to be treated in a chemical recovery process. Further economies in recovery of this filtrate can be achieved if a portion of this filtrate is used for dilution of pulp which is to be oxygen gas treated. This has been found to have no deleterious efiects on the brightnesses obtained.

Such filtrate, or recirculated filtrate, may then, for example, be used as portion of the wash water used in the normal washing stages applied to the pulp as produced by the pulping process concerned, or may be added directly to the liquors at any stage in the recovery process concerned. Alternatively, the caustic soda in the filtrate may be recovered by any of the known methods of recovery. It is a big advantage of this process that despite the load of organic matter in these filtrates, they are substantially free of chlorides which are normally always present in bleach plant effluents, apart from any chloride that might be introduced when the protector contains the element chlorine. This enables the chemical recovery process to be conducted readily, without the serious interferences caused by chlorides, and allows, if necessary, any recovered sodium carbonate to be converted to caustic soda for repeated re-use. Even if, for instance, magnesium chloride is used as the protector, the amount of chlorides will still be considerably less than would be the case for conventional bleaching sequences. The very heavy pollution load caused by normal multistage bleaching operations will in this way be very beneficially reduced.

The following examples are illustrative of the invention without in any way limiting the scope thereof. The concentrations are given as percentages by weight on a dry pulp basis, except for sodium silicate, the amount of which is based on the solids content of the commercial product. The oxygen pressures are those pertaining at the temperature stated. Pulp consistency is the ratio, expressed as a percentage, of the weight of the dry pulp to the weight of the wet pulp. All tests for the examples were carried out in accordance with TAPPI or AFNOR specifications.

EXAMPLE I The lignin content, the brightness and the physical strength prpperties of the following pulps are compared in Table l e ow:

a. untreated, unbleached, resinous Kraft pulp (hereinafter referred to as Pulp A).

b. the pulp of (a) above after oxygen gas treatment in an alkaline medium without a protector (hereinafter referred to as Pulp B) c. the pulp of (a) above after oxygen gas treatment in an alkaline medium in the presence of magnesium carbonate which is the preferred protector disclosed in our French Patent Specification No. 1,387,853, (hereinafter referred to as Pulp C).

d. different batches of the pulp of (a) above after oxygen gas treatment in alkaline medium in the presence of different protectors according to the present invention (hereinafter referred to as Pulps D.,E.F. and G respectively.)

It will be seen that Pulps D,E,F and G which have been treated in the presence'of protectors according to the present invention, have higher brightnesses than pulps AB and C. It

will also be seen in relation to pulps D,E,F and G that the protectors according to the present invention effectively preserve the physical strength properties of the pulp and even improve them in certain respects.

We claim:

1. A method of delignifying and bleaching a chemical or semi-chemical cellulose pulp, said method consisting essentially of subjecting the pulp to treatment with oxygen gas in an EXAMPLE H alkaline medium in the presence of up to 3 percent by weight I of a protector which is operative to inhibit degradation of the The hgnin content, the brightness and the physical strength physical and mechanical strength properties of the pulp and properties of the following pulps are compared in Table II which comprises a substance other than that constituting the below: 1 O alkaline medium, said protector being selected from the group a. Untreated unbleached, pine kraft pulp (hereinafter consisting of silica; an alkali metal silicate; an alkaline earth referred to as Pulp H) metal silicate; an alkaline earth metal phosphate; an alkaline b. The pulp of (a) above after oxygen gas treatment in an alearth metal oxide; an alkaline earth metal peroxide; an alkaline medium without a protector (hereinafter referred kaline earth metal hydroxide; and a soluble salt of magnesium to as Pulp I) l 5 other than magnesium carbonate. c. Different batches of the pulp of (a) above after oxygen 2. A method as claimed in claim 1 wherein the protector gas treatment in alkaline medium in the presence of difcomprises sodium silicate. ferent protectors according to the present invention 3. A method as claimed in claim 1 wherein the protector (hereinafter referred to as Pulps J and K respectively) comprises magnesium oxide. It will be seen that the brightnesses of Pulps J and K which 4. A method as claimed in claim 1, wherein the protector have been treated in the presence of protectors according to comprises magnesium silicate. the present invention, are comparable with that of Pulp I and 5. A method as claimed in claim 1, wherein the protector much higher than that of Pulp H. The protectors used on pulps comprises magnesium hydroxide. .l and K effectively preserve the physical strength properties of 6. A method as claimed in claim 1, wherein the protector the pulp and even improves them in certain respects. comprises magnesium peroxide.

TABLE I.RES1NOUS PINE KRAFT PULP AFNOR STANDARD Pulp A B c D E F G Pulp consistency 15 15 15 15 15 15 Alkaline medium (percent NaOH). 5 5 5 5 5 0 Protector, percent 0 Oxygen pressure at ambient temp. (p.s.i.g.). 100 100 100 100 100 105 Maximum operating temp. C.) 120 120 120 120 120 120 Time at max. operating temp. (minutes) 30 30 30 30 3O Brightness (photovolt) 54 52 56 58 57 55 KMnOi number"... 7 6.4 5.1 4.9 4.6 6.3 Sr 29 32 32 32 30 29 Time of beating (minutes) 45 45 45 45 45 Breaking length (metres) 7,300 6, 600 7,800 8,300 9,200 7,000 7,100 Double folds (under 885 gr.) 1,500 117 700 540 1,000 590 1,066 Burst factor 57. 5 43. 2 51. 5 58 53. 5 51 54. 7 Tear factor 102 70. 6 72 80. 5 85.6 81 84 1 1% M co 1 1% MgO. 3 1% NazSiOa. 4 1% M sioi. 1% SiOz. 7 7 A 4 TABLE H- PINE KRAFT PULP TAPPI SPECIFICATIONS 7. A method as claimed in claim 1, wherein the protector is Pulp H I J K 45 used at a concentration of from 0.5 to 3 percent by weight on a dry pulp basis. Pulp consistency 17 15 17 Alkaline medium (percent NaOH per- 8. A method as claimed in claim 1, wherein the alkaline cent) 6 5 6 Protector, percent N11 i 2) medlum compnses Soqmm h x Oxygen pressure at maximum operat- 9. A method as claimed in claim 7, wherein the protector ing temp., p.s.i. gauge 170 170 170 Maximum operating temp. C.) 130 130 130 compnses 1 percent by W l on a pulp Time at maximum operating tempera- 10. A method as claimed in claim 8, wherein the SOdlUm ture (minutes) 30 30 30 Brightness (Elmpho Zeiss) 0 413 42 7 42 2 hydroxide complflses from 2 to 12 Percent by Weight of the Ki\1nOi iiumher 'KAPPA (AFNO R). 26.3 4. 2s 4. 0 4. 2 pulp on a dry basis. C.S.F 595 270 465 430 Revs. of heating 0, 000 J, 000 0, 000 0,000 Breaking length (metres) 10, 000 9, 810 10,000 10,700 Double folds (under 800 gr.) A 712 467 722 776 Burst factor 64. 4 58. 0 67. 7 68. 2 Tear iaetor 107. 0 70. d 91. 7 90. ll

1 1% Mg(OII)2. 2 1 4% MgCla. 

2. A method as claimed in claim 1, wherein the protector comprises sodium silicate.
 3. A method as claimed in claim 1, wherein the protector comprises magnesium oxide.
 4. A method as claimed in claim 1, wherein the protector comprises magnesium silicate.
 5. A method as claimed in claim 1, whErein the protector comprises magnesium hydroxide.
 6. A method as claimed in claim 1, wherein the protector comprises magnesium peroxide.
 7. A method as claimed in claim 1, wherein the protector is used at a concentration of from 0.5 to 3 percent by weight on a dry pulp basis.
 8. A method as claimed in claim 1, wherein the alkaline medium comprises sodium hydroxide.
 9. A method as claimed in claim 7, wherein the protector comprises 1 percent by weight on a dry pulp basis.
 10. A method as claimed in claim 8, wherein the sodium hydroxide comprises from 2 to 12 percent by weight of the pulp on a dry basis. 